Bonded abrasive and method of making same



, Jan. 7, 1947.l R H, RUSHMER l l2,413,729

BONDED ABRASIVE AND METHOD OF MAKING SAME Filed March '7, 1944 Patented7, 1947 BONDED ABRASIVE AND METHOD OF MAKING SAME Ralph H. Rushmer,Niagara Falls, N. Y., assigner to The Carborundum Company, NiagaraFalls,

N. Y., a corporation of Delaware Application March 7, 1944, Serial No.525,379

' 11 claims. (c1. 51-1298') This invention relates to resinoid bondedabrasive articles and to methods of making them. More particularly, theinvention relates to an improved method of molding and curing resinoidbonded abrasive articles and to the article resulting therefrom.

Among the objects of this invention is the provision of a method ofmolding and curing resinoid bonded abrasive articles whereby such curedarticles are usable in their entirety and need not be further finishedto size. Another object of this invention is the provision of a methodwhich eliminates sticking of the molded article to its support duringcuring. A still further object is the production of a molded resinoidbonded abrasive article having distinctive surface characteristics. Afurther object is the production of an improved resinoid bonded abrasivearticle having uniform characteristics throughout and accurately andeconomically molded to final thickness. These and further objects oftheinvention will appear as the description proceeds.

In making resinoid bonded abrasive articles, such as for instancewheels, in accordance with prior art methods, a suitable mixture ofabrasive grit and resinoid bond is putinto a mold of the desired shape,pressed to a suitable density, and then cured either by heating in themold or by taking the pressed article from the mold, putting it on abatt, surrounding it with inert granular material to prevent slumping,and curing the the curing to iiow to a certain extent to the outer facesof the article.

In addition to the defacing of the article caused by the aforementionedsticking to the support during cure, the articles produced by prior artmethods which are molded to final thickness are non-uniform incharacter. The outer portions of the cured articles, more particularlythe broad faces, have such different structure from that in the interiorof the article that if a wheel so made is used in a snaggingoperation in its cured condition without being faced, a spalling orshucking-oif of the outer layer on the faces, more particularly at thecorners, occurs in a relatively short time. Such spalling in a wheel11/2 inches thick may, for example,extend inwardly to a depth of M3"from each face'.

The reasons for this phenomenon are not fully known. It has beentheorized that, since the cure takes place in an oven which, althoughclosed, is not sealed, and had airin it at the start pressed article inan oven by heating it for a relatively long time in accordance with apredetermined cycle of curing. The curing of the pressed article in themold is not economical since for quantity production this requires alarge number of relatively expensive molds and so the usual procedure isto cure such articles on batts. In

either prior art curing method, however, there are inherent diilicultieswhich, until the present invention, have been accepted as linevitable bythe industry. If the pressed article is cured in the mold, it tends tostick to the top and bottom plungers thereof, at least in. portions, sothat upon withdrawal of the cured article from the mold portions aretorn from the faces of the article. If the article is curedby removingit from the mold and heating while on a batt, it also sticks to the battand upon removal therefrom at the end of the cure tends to tear portionsfrom the batt. Such material from the batt must be removed from themolded article by facing the same. The tendency of the pressedarticle-to stick to either the mold or the batt appears to be aggravatedby the tendency of the resin during of the cure, oxidation of the resinbond takes place on the exposed surfaces of the article. According tothis theory the oxidized resin lacks the strength of the resin in theinterior of the article and it soon becomes weakened and broken underoperating conditions. Another theory is that since a relatively largeamount of resin solvent, which may be, for instance, furfural, is

given ofi by the articles during cure and the atmosphere in the ovencontains a relatively large amount of such solvent, the resin on theexposed portions of the article, being constantly subjected to suchsolvent atmosphere, is of a different type from that in the interior ofthe article and possesses different properties, such as a diiferentcoeicient of expansion. The adherents of this theory believe that uponheating of a resin bonded abrasive article so produced, due to its usein a severe grinding operation, the difference in coefficients ofexpansion between the inner and outer portions of the article causes abreak age to occur in the boundary between such portions, which is aplanev more or less parallel to the surface, thus resulting in theobserved spalling.

Under a relatively low power microscope the surface of a resin bondedabrasive article resulting from a prior art cure on a batt in an ovenshows numerous small shallow dark patches in 'the resin where such resincommunicates with the surface, whereas the cured resin in the interiorof the article is a greenish gray in color. This clearly shows that theresin on the exterior of such article is of a diiferent character frommaking a wheel suitable for snagging operations.

` A mixture of abrasive grit and bond resin, having the followingingredients in the proportions by weight indicated, is made up in theconventional manner:

. Bakelite resin BR 1372 is an unmodified condensation product of thereaction between phenol and an aldehyde. Bakelite resin BR 2417 islikewise an unmodified condensation product of the reaction betweenphenol and an aldehyde, but is advanced to a solid state by heating.

Such abrasive resin mixture is charged into a mold such as that shown inFigure 2. Protecting layers I4 and I6 in this instance consist of filmsof ethyl cellulose approximately .001 thick. After being molded-toshape, the wheel is heated in an oven which is raised from 70 to 375 F.In a period of 24 hours, is held at 375 F. for hours, and is then cooledto room temperature at a rate of approximately 20 F. per hour. It isobvious that` other synthetic resin bonds may be employed in thepractice of this invention if desired, such resinsbeing such that theysoften somewhat upon initiation of the curing treatment. If other resinsare used the curing cycle chosen for the molded article is one which isappropriate to the resin.v

The ethyl cellulose film employed in this example is one known asEthofoil No. 10, a product of the Dow Chemical Company. This material,which has a softening temperature of 270,

F. and a melting temperature of 330 F., has proved to be ideal incarrying out the method of the*k present invention'. Upon completion ofthe cure of the abrasive article the ethyl cellulose lm is found to beblackened and charred so that it can be easily removed, indicating thatit has almost been completely decomposed.

In place of ethyl cellulose films other films or layers of thermoplasticmaterials which soften at temperatures well below the maximumtemperature of the cure may be used. Preferably, such layers or filmsdecompose at least partially under the curing conditions employed.Cellulose derivatives, that is, cellulose esters or cellulose ethers,have been found t be ideal for this purpose. One material of thischaracter, other than the ethyl cellulose already mentioned, iscellulose acetate. This material, which melts at from 455-555 F.,depending upon the degree of acetylation, but which is thermoplastic atmuch lower temperatures, remains on the cured article as a more or lesscontinuous layer although some darkening of the layer is observed,showing that some decomposition of such layer has occurred. Celluloseacetate containing from between 50-58% acetic acid and having a meltingpoint from 455518 F. may be employed. It has been found that thecellulose acetate designated as type F, made by the Hercules 'PowderCompany, which contains from 5557% acetic acid is eminentlysatisfactory. Such material may be used in any of the three viscositiesin which itis made, namely, low, below 20 seconds, medium, 20 seconds to60 seconds, and high, over 60 seconds.

Another cellulose derivative which may be use'd in the present inventionis cellulose nitrate. This may be applied as a lmin the same manner asin the case of the two materials mentioned above.

6 Among other materials suitable for the protecting layers or films onAmolded resinoid bonded articles are polystyren cellulose acetylbutyrate, polyvinyl alcohol, polyvinyl chloride, polyvinyl butyral,polyvinyl chloro-acetate. vinylidene chloride, methyl methacrylate. andacrylic copolymers.

Although the application of films orthin layers of such protectingmaterials has been disclosed as being accomplished by cutting outsuitably sized pieces from preformed sheets or lms of such material andapplying them to the article, it is obvious vthat such thin layers maybe applied, where feasible, by painting or spraying such material in asuitable vehicle onto the molded article or onto th`e mold part orsupport u with whichit comes in contact. Thus, protecting layer I4 mightbe applied to mold part l by spraying such material in a vehicle ontothe mold part, which has been suitably coated with a parting layer suchas an oil. to a suitable thickness, and protective layer IB might beapplied to mold part 8, similarly previously coated with a partingmaterial, by a similar process. Upon compression of the abrasive-resinmixture in the mold between parts I and 8, protecting layers I4 and I6become attached to the molded article and upon stripping of the moldedarticle from the mold such layers remain on the article. The article maybe molded either hot or, cold in the conventional manner and theprotecting layers applied later either as preformed lms as alreadyindicated, or such layers may beapplied to the molded article by meansof painting or spraying the article with the materials above dissolvedor dispersed in a suitable vehicle.

Although in order to attain uniform structure throughout the moldedarticle it is preferred that protecting layers be placed on both suchfaces, one of the advantages of the present invention, namely theprevention of sticking of the molded article to the support duringcuring,

may be accomplished by use of only'one protecting layer which isinterposed between the molded article and the support upon which itrests during curing. The protecting layers employed may range inthickness from one which is handleable with reasonable care, in theorder of 0088" to one of appreciable thickness,l such as .01" or over.

The method of the present invention nds particular-advantage, insofar aspreventing sticking to the' support during curing is concerned, in thecoarse grit sizes such as those used in -snagging wheels. such gru:sizes may be said to be roughly those including and lying between 12 and36 grit. In all resin bonded articles molded to finished thickness andcured by prior art methods, however, a difference in structure betweenthe inner and outer parts has been apparent. The present inventionproduces cured resin bonded 'abrasive article which are uniformthroughout, when coatings of the type disclosed are used on both broadfaces of the article. It is to be understood, therefore, that thepresent invention is not limited to the employment of coarse grits andthat it displays advantages both in producing articles of uniformstructure and preventing sticking of the molded articles to the supportson which they are cured regardless of the particular size or sizes ofgrit employed.

While preferred examples of the method of molding and curing resinoidbonded granular atraveo su or die describes cereus out met a may seotherwise embodied within the scope or the iollowina claims. j

i claim:

i. The method oi making resin bonded abrasive articles of substantialthickness having the resin bond uniformly cured throughout comprising'molding the article directly to the desired thickness from a mixcomprising abrasive grain and a resin hond therefor which softens duringthe heat treatment of the article, incorporating on each of the sidefaces of the article prior to the curingof the resin hond a thin layerof the order or not morathan about a hundredth of an inch thickness of asynthetic plastic material o permanently thermoplastic character toprotect the hond of the article from external iniiuences such asoxidation during the heat treatment thereof 'and to provide nal sidesurfaces for the nished article, heat treating the formed article whilethus protected to mature the resin hond uniformly from one sidethroughout the article to the opposite side and adhere the plasticmaterial to the surface of the article in the form of substantiallycontinuous films as a protection thereto while said article is supportedon a rigid member with one ofthe layers of plastic material positionedbetween the formed article and the supporting member, and removing saidabrasive article from said supporting member.

2. The method of making resin bonded abrasive articles of substantialthickness having the resin4 bond uniformly cured throughout comprisingmolding the article directly to the desired thickness from a mixcomprising abrasive grain and a resin bond therefor which softens duringthe heat treatment of the article, incorporating on each of the sidefaces of the article prior to the curing of the resin bond a thin layerci the order of not more than about a hundredth of an inch thickness ofa' synthetic plastic material of thermoplastic character selected fromthe group consisting of cellulose esters and ethers and permanentlythermoplastic synthetic resins to protect the bond of the article fromexternal influences such as oxidation during the heat treatment thereofand to provide nal side surfaces for the nished article, heat treatingthe formed article while thus protected to mature the resin bonduniformly from one side throughout the article -f 8l to the oppositeside and adhere the plastic material to the suriacesofthe article in theform oi substantially continuous films as a, protection thereto whilesaid article is supported on a rigid member with one of the layers ofplastic material positioned between the formed article and thesupporting member, and removing said abrasive article from saidsupporting member.

3. A resin bonded abrasive article comprising a mass of abrasive grainsbonded with a heat hardened resin, said resin bond structpre andcomposition being the same on the exterior side faces or the article asthe resin bond structure and composition of the interior of the article,the said side iaces of said article constitutingl the originally moldedfaces of the article, said article having the side faces protected by a.substantially continuous, thin film in the order of not more than abouta hundredth of an inch thickness oi an initially thermoplastic,synthetic plastic material in at least partially decomposed or charredcondition and conforming to the contour ofthe abrasive grainsconstituting said faces of the article.

d. The method set out in claim 1 in which said thermoplastic material iscellulose acetate.

5. The method set out in claim 1 in which said thermoplastic material isethyl cellulose.

6. The method set out in claim 1 in which said thermoplastic material iscellulose nitrate.

7. The method as set forth in claim 1 in which said thermoplasticmaterial is a cellulose derivative. v

8. A resin bonded article as set out in claim 3 in which said initiallythermoplastic material is heat treated and at least partially decomposedcellulose acetate.

9. A resin bonded article as set out in claim 3 in which said initiallythermoplastic material is .heat treated and at least partiallydecomposed V in which said initially thermoplastic material is heattreated and at least partially decomposed cellulose nitrate.

11. A resin bonded article as ser. out 1n claim 3 ,f in which saidinitially thermoplastic material is i RALPH H. RUS. 1

